Group iii nitride based flip-chip integrated circuit and method for fabricating

ABSTRACT

A circuit substrate has one or more active components and a plurality of passive circuit elements on a first surface. An active semiconductor device has a substrate with layers of material and a plurality of terminals. The active semiconductor device is flip-chip mounted on the circuit substrate and at least one of the terminals of the device is electrically connected to an active component on the circuit substrate. The active components on the substrate and the flip-chip mounted active semiconductor device, in combination with passive circuit elements, form preamplifiers and an output amplifier respectively. In a power switching configuration, the circuit substrate has logic control circuits on a first surface. A semiconductor transistor flip-chip mounted on the circuit substrate is electrically connected to the control circuits on the first surface to thereby control the on and off switching of the flip-chip mounted device.

RELATED APPLICATIONS

This application is a divisional from, and claims the benefit of, U.S.patent application Ser. No. 12/951,372, having the same title, with is adivisional from, and claims the benefit of, U.S. patent application Ser.No. 10/977,165, having the same title and filed on Oct. 29, 2004, whichis a continuation-in-part from, and claims the benefit of, U.S. patentapplication Ser. No. 10/335,915, filed Jan. 2, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor devices, and more particularlyto semiconductor devices that are flip-chip mounted on circuitsubstrates having passive components and/or active components.

2. Description of Related Art

Microwave systems commonly use solid state transistors as amplifiers andoscillators, which has resulted in significantly reduced system size andincreased reliability. To accommodate the expanding number of microwavesystems, there is an interest in increasing their operating frequencyand power. Higher frequency signals can carry more information(bandwidth), allow for smaller antennas with very high gain, and provideradar with improved resolution.

Field effect transistors (FETs) and high electron mobility transistors(HEMTs) are common solid state transistors that can be fabricated fromsemiconductor materials such as silicon (Si) or gallium arsenide (GaAs).One disadvantage of Si is that it has low electron mobility(approximately 1450 cm2/V-s), which produces a high source resistance.This resistance seriously degrades the high performance gain otherwisepossible from Si based FETs. [CRC Press, The Electrical EngineeringHandbook, Second Edition, Dorf, p. 994, (1997)]

GaAs is also a common material for use in HEMTs and has become thestandard for signal amplification in civil and military radar, handsetcellular, and satellite communications. GaAs HEMTs have a higherelectron mobility (approximately 6000 cm²/V-s) and a lower sourceresistance than Si, which allows GaAs based devices to function athigher frequencies. However, GaAs has a relatively small bandgap (1.42eV at room temperature) and relatively small breakdown voltage, whichprevents GaAs based HEMTs from providing high power.

Improvements in the manufacturing of Group-III nitride basedsemiconductor materials such as gallium nitride (GaN) and aluminumgallium nitride (AlGaN) has focused interest on the development ofAlGaN/GaN based devices such as HEMTs. These devices can generate largeamounts of power because of their unique combination of materialcharacteristics including high breakdown fields, wide bandgaps (3.36 eVfor GaN at room temperature), large conduction band offset, and highsaturated electron drift velocity. The same size AlGaN/GaN amplifier canproduce around ten times the power of a GaAs amplifier operating at thesame frequency.

U.S. Pat. No. 5,192,987 to Khan et al. discloses AlGaN/GaN based HEMTsgrown on a buffer and a substrate, and a method for producing a HEMT.Other HEMTs have been described by Gaska et al., “High-TemperaturePerformance of AlGaN/GaN HFET's on SiC Substrates,” IEEE Electron DeviceLetters, Vol. 18, No 10, October 1997, Page 492; and Wu et al. “HighAl-content AlGaN/GaN HEMTs With Very High Performance”, IEDM-1999 Digestpp. 925-927, Washington D.C., December 1999. Some of these devices haveshown a gain-bandwidth product (f_(T)) as high as 100 gigahertz (Lu etal. “AlGaN/GaN HEMTs on SiC With Over 100 GHz ft and Low MicrowaveNoise”, IEEE Transactions on Electron Devices, Vol. 48, No. 3, March2001, pp. 581-585) and high power densities up to 10 W/mm at X-band (Wuet al., “Bias-dependent Performance of High-Power AlGaN/GaN HEMTs”,IEDM-2001, Washington D.C., Dec. 2-6, 2001)

Group-III nitride based semiconductor devices are often fabricatedeither on sapphire or SiC substrates. One disadvantage of sapphiresubstrates is that they have poor thermal conductivity and the totalpower output of devices formed on sapphire substrates can be limited bythe substrate's thermal dissipation. Sapphire substrates are alsodifficult to etch. SiC substrates have higher thermal conductivity(3.5-5 w/cmk) but have the disadvantages of being relatively expensiveand not available in large wafer diameters of 4-inch and greater.Typical semi-insulating SiC wafers are three inches in diameter and ifthe active layers of a transistor are formed on the wafer along with thepassive components, interconnections, and/or pre-stage amplifiers, theyield in number of devices per wafer is relatively low. This reducedyield adds to the cost of fabricating Group III transistors on SiCsubstrates.

GaAs and Si semi-insulating wafers are available in larger diameters ata relatively low cost compared to the smaller diameter SiC wafers. GaAsand Si wafers are easier to etch and have low electrical conductivity.Another advantage of these wafers is that fabrication of semiconductordevices and other processing can be conducted at a commercial foundry,which can reduce cost. One disadvantage of these wafers is that theycannot be easily used as a substrate for Group-III nitride based devicesbecause the lattice mismatch between the materials leads to poor qualitysemiconductor devices. Another disadvantage of these wafers is that theyhave low thermal conductivity.

In some instances GaAs semiconductor devices may not have the necessaryoperating characteristics. For example, a GaAs based multistageamplifier may not be able to provide the required output power levels.In such cases it may be desirable to fabricate a GaN based multistageamplifier. However, such an implementation has some disadvantages.First, the area required for a multi-stage MMIC with interstage networksand all the passive circuit elements is large. Expensive real estate onthe GaN transistor substrate would be used up in hosting passiveelements. Second, device and circuit models of GaN transistors are lessestablished than their GaAs counterparts, thus making it difficult toaccurately design and yield multistage GaN amplifiers that providehigher gain and power outputs.

SUMMARY OF THE INVENTION

A flip-chip integrated circuit according to the invention includes acircuit substrate having one or more active components and a pluralityof passive circuit elements on a first surface. The circuit alsoincludes an active semiconductor device comprising a substrate withlayers of material and a plurality of terminals. Each of the terminalsis in electrical contact with one of the layers of material. The activesemiconductor device is flip-chip mounted on the circuit substrate andat least one of the terminals of the device is electrically connected toan active component on the circuit substrate.

In another configuration, a flip-chip integrated circuit includes acircuit substrate with amplifier electronics and passive circuitelements on a first surface and a semiconductor transistor that isflip-chip mounted on the circuit substrate. The flip-chip transistor isin electrical connection with passive circuit elements on the firstsurface. The flip-chip transistor, in combination with passive circuitelements, forms a flip-chip amplifier having an input electricallyconnected to the amplifier electronics.

Another flip-chip integrated circuit according to the invention includesa circuit substrate having control circuits on a first surface and asemiconductor transistor that is flip-chip mounted on the circuitsubstrate. Another control circuit configuration includes a Si basedcircuit substrate with control circuit elements on a first surface and aGroup III nitride semiconductor transistor that is mounted on thecircuit substrate. The transistor has a gate and a drain, at least oneof which is electrically connected to one of the control circuitelements on the first surface. The circuit can be used for powerswitching applications

In another flip-chip integrated circuit configuration, a circuitsubstrate including an insulating material, has passive circuit elementson a first surface. A Group III nitride active semiconductor device isflip-chip mounted on the circuit substrate and is electrically connectedto the passive circuit element.

Another flip-chip integrated circuit according to the invention includesa circuit substrate having drive electronics on a first surface andconductive material on a second surface opposite the first surface. Anactive semiconductor device having a plurality of terminals on a firstsurface and a layer of material on a second surface opposite the firstsurface, is flip-chip mounted on the circuit substrate so that at leastone of its terminals is in electrical contact with the drive electronicson the circuit substrate. The circuit further includes a first heat sinkadjacent the conductive material on the circuit substrate and a secondheat sink adjacent the layer of material on the active semiconductordevice.

These and other further features and advantages of the invention wouldbe apparent to those skilled in the art from the following detaileddescription, taking together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram for a method of fabricating an integratedcircuit according to the present invention;

FIG. 2 is a plan view of a SiC wafer according to the present invention,with AlGaN/GaN HEMTS formed on it;

FIG. 3 is a sectional view of two of the AlGaN/GaN HEMTs formed on thewafer shown in FIG. 2;

FIG. 4, is a sectional of an individual HEMT separated from the otherHEMTs on the wafer of FIG. 2;

FIG. 5 is a sectional view of a circuit substrate according to thepresent invention;

FIG. 6 is a sectional view of a second circuit substrate according tothe present invention;

FIG. 7 is a sectional view of a third circuit substrate according to thepresent invention;

FIG. 8 is a sectional view of a fourth circuit substrate according tothe present invention;

FIG. 9 is a sectional view of an integrated circuit according to thepresent invention with the circuit substrate having a HEMT flip-chipmounted to it;

FIG. 10 is a sectional view of the device in FIG. 8 with a first heatsink root on the bottom surface of the circuit substrate;

FIG. 11 is a sectional view of the device in FIG. 9 with a second heatsink root adjacent to the substrate of the HEMT;

FIG. 12 is a sectional view of another integrated circuit according tothe present invention with a flip-chip mounted HEMT and second heat sinkroot;

FIG. 13 is a schematic diagram of a multistage amplifier;

FIG. 14 is a sectional view of an integrated circuit according to thepresent invention including some of the components of the amplifiercircuitry of FIG. 13;

FIG. 15 is a schematic diagram of a power switching circuit; and

FIG. 16 is a sectional view of an integrated circuit according to thepresent invention including some of the components of the powerswitching circuitry of FIG. 15;

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one embodiment of a method 10 according to the presentinvention. In the first step 12, semiconductor layers of a semiconductordevice and device terminals are formed on a wafer. A preferredsemiconductor device is a Group III nitride based device such as anAlGaN HEMT or FET grown on a sapphire, SiC, GaN, AlN or Si wafer, withthe preferred wafer being a 4H polytype of SiC. Other SiC polytypes canalso be used including 3C, 6H and 15R polytypes. An Al_(x)Ga_(1-x)Nbuffer layer (where x in between 0 and 1) can be included between thewafer and device active layers to provide an appropriate crystalstructure transition between the SiC wafer (substrate) and the activelayers.

Generally, SiC wafers are preferred over sapphire and Si because theyhave a much closer crystal lattice match to Group III nitrides, whichresults in Group III nitride films of higher quality. SiC also has avery high thermal conductivity so that the total output power of GroupIII nitride devices on SiC is not limited by the thermal resistance ofthe wafer (as is the case with some devices formed on sapphire or Si).Also, the availability of semi insulating SiC wafers provides thecapacity for device isolation and reduced parasitic capacitance thatmake commercial devices possible. SiC substrates are available from CreeInc., of Durham, N.C. and methods for producing them are set forth inthe scientific literature as well as in a U.S. Pat. Nos. Re. 34,861;4,946,547; and 5,200,022. Other possible substrate materials includegallium nitride (GaN) and aluminum nitride (AlN).

The Al_(x)Ga_(1-x)N and other epitaxial layers can be deposited on thewafer using different epitaxial growth methods such as metalorganicchemical vapor deposition (MOCVD), plasma chemical vapor deposition(CVD) or hot-filament CVD or Molecular Beam Epitaxy (MBE). The terminalscan be deposited on the active layers using sputtering, evaporation orplating, including other steps as necessary to complete devicefabrication.

For a HEMT, the terminals include source and drain contacts thatpreferably comprise alloys of titanium, aluminum, nickel and gold, and agate contact that preferably comprises titanium, platinum, chromium,nickel, gold, alloys of titanium and tungsten, and platinum silicide.

In step 14, bonding pads are formed on at least one of the device'sterminals and the bond will contact the circuit substrate when thedevice is flip-chip mounted as described below. During typical operationof an AlGaN HEMT the drain contact is biased at a specified potential(positive drain potential for an n-channel device) and the source isgrounded. For HEMTs, the bond pad is included on the source contact soit can be electrically connected to ground on the circuit substrate. Thebonding pads are preferably made of a highly conductive material such asgold (Au) and they can be deposited using sputtering. Other materialscan be used such as a solder bonding pad and bond pads can also beprovided on other contacts of the HEMT as required.

In step 16 the active semiconductor devices on the wafer are separatedinto individual devices, preferably by dicing. Alternatively the devicescan be separated by a scribe and break. If needed, parts of the wafercan also be separated instead of individual devices.

In step 18 drive electronics for driving one or more of the activesemiconductor devices are formed on a circuit substrate wafer. Thecircuit substrate should be low cost, available in large diameters, easyto process, have low electrical conductivity and high thermalconductivity. GaAs and Si are suitable materials for the circuitsubstrate and have all of the desired characteristics except highthermal conductivity. Other suitable circuit substrate materials includeSiC, indium phosphide (InP) and insulating materials such as aluminumnitride (AlN) and ceramic materials. The thermal conductivity of thesematerials can be improved by using conductive vias as described below.The drive electronics can comprise active components, passive componentsand interconnections in different combinations. In one such combination,the active components include transistors. These transistors, incombination with one or more of the passive components, may beinterconnected to form a pre-stage amplifier. In another combination,the active components may form logic circuits.

The drive electronics form drive circuits for the flip-chip mountedactive semiconductor devices. In embodiments including pre-stageamplifiers, the amplifiers are typically interconnected in series toamplify lower signals, thereby forming a multistage amplifier. After thepre-stage amplifiers amplify the signal, it can be applied to theflip-chip mounted semiconductor device for high power amplification. Inthis configuration, the flip-chip mounted semiconductor device is atransistor that is electrically connected to passive components on thecircuit substrate to form an amplifier. Pre-stage amplifiers are notneeded in embodiments where the flip-chip mounted device can be drivenby passive components and interconnects without pre-stage amplification.The active components, passive components and interconnections can bedeposited on the circuit substrate using a commercial foundry process,which helps reduce manufacturing costs.

A different number of active devices can be driven by the driveelectronics. In one embodiment, a single drive electronics circuit candrive a single active device. In other embodiments, a drive circuit candrive more than one active device or one active device can be driven bymore than one drive electronics circuit.

The passive components can include, but are not limited to, resisters,capacitors and inductors, and the interconnections can comprise tracesof conductive material or transmission line network elements on thecircuit substrate. The active components and passive components can beformed using MOCVD, CVD or hot-filament CVD, MBE and the traces can beformed using sputtering or electron beam deposition or plating.

In alternative step 20, one or more conductive vias are formed throughthe circuit substrate, with each of the drive circuits utilizing atleast one conductive vias. In one embodiment according to the invention,the vias form a conductive path to ground for an active semiconductordevice that is flip-chip mounted to one of the drive circuits asdescribed below and they also promote the device's heat dissipation.Different methods can be used for forming the vias including but notlimited to forming a hole through the circuit substrate using wetchemical HF etching, RIE, ICP or plasma etching. The inside surface ofthe vias can then be covered with a layer of conductive material,preferably gold (Au), which can be deposited using sputtering. In stillanother embodiment, the top of the conductive vias can include a plug ofconductive material to enhance heat dissipation.

In another embodiment of a method according to the present invention,the circuit substrate for active flip-chip devices can operate withoutthe path through the substrate and as a result, the device does notinclude conductive vias or plug. The device can be connected to groundthrough other paths, such as through a coplanar waveguide arrangementincluding interconnects on the circuit substrate, and heat can beextracted from the device in other ways, such as through a heat sinkattached to the back of the device.

In step 22, the active semiconductor device is flip-chip mounted on thecircuit substrate having the Au bond pad in electrical contact with theAu in the vias, in an Au—Au flip chip bond. Alternatively, conventionbump bonding based on Au or solder can be used. For an AlGaN HEMT, theAu bond on the source contact is in electrical contact with the vias.The gate and drain contacts can then be electrically bonded to the driveelectronics on the circuit substrate, with the gate typically connectedto the input side and the drain connected to the output side of thedrive electronics.

In step 24 the drive electronic circuits and active devices on thecircuit substrate are separated into individual integrated circuits.This can be accomplished by the same methods described above that areused to separate the active devices or regions of active devices.

In another alternative step 26, one or more heat sink base plates can beformed on the integrated circuit (before or after being separated), withthe base plates then being connected to one or more heat sinks. Heatfrom the active device and circuit substrate flows into the base platesand then into the heat sink, where it dissipates. The base plates can bearranged in many different ways, including but not limited to, adjacentto the active device and/or the circuit substrate.

Methods according to the present invention can be used to fabricate manydifferent devices beyond AlGaN HEMTs. The different steps of the methodscan also be accomplished using different processes and the steps of themethods can be taken in different order.

The present invention also discloses an active semiconductor device thatis flip-chip mounted on a circuit substrate having active componentsand/or passive components and interconnections. FIG. 2 shows a typicalsemi insulating SiC wafer 30 that is available in different diameters,including approximately two or three inches. The device active layersand terminals 32, shown as squares on the wafer 30, are deposited on thewafer 30 using the methods described above. The illustration is only arepresentation of the number of devices that can be formed on a wafer.The preferred device active layers and terminals 32 form an AlGaN HEMTand for a typical 2-inch wafer with 10-Watt HEMTs, approximately 2000HEMTs can be formed on the wafer. If the passive components or pre-stageamplifiers were formed on the wafer with the HEMT, only approximately200 devices could be formed.

FIG. 3 shows a sectional view of two of the AlGaN/GaN based HEMTs 32formed on the wafer 30 according to the present invention. When theHEMTs 32 are separated into individual devices, the portions of thewafer 30 that remain with each HEMT serve as the HEMT's substrate. AnAl_(x)Ga_(1-x)N buffer layer (where x in between 0 and 1) (not shown)can be included between the wafer and device active layers to providethe desired crystal structure transition between the wafer and theactive layers.

A GaN high resistivity layer 34 is deposited on the wafer 30 and a AlGaNbarrier layer 36 is deposited on the high resistivity layer 34. The highresistivity layer 34 is typically about 0.5 to 4 micrometers thick andthe barrier layer 36 is typically about 10 to 40 nm thick.

To provide separation between the individual HEMTS and to provide alocation for the source and drain contacts 38, 40, the barrier layer 36is etched to the high resistivity layer 34. The source and draincontacts 38, 40 are deposited on the surface of the high resistivitylayer 34 with the barrier layer 36 disposed between them. Each of thecontacts 38, 40 are in electrical contact with the edge of the barrierlayer 36.

The contacts 38, 40 are usually separated by a distance in the range 1.5to 5 micrometers for microwave devices, but could be 1 to 10 micrometersin special cases. A rectifying Schottky contact (gate) 42 is located onthe surface of the barrier layer 36 between the source and draincontacts 38, 40, and it typically has a length in the range of 0.1 to 2micrometers. The total width of the HEMT 32 depends on the total powerrequired. It can be wider than 30 millimeters, with the typical widthbeing in the range of 100 microns to 6 millimeters.

The barrier layer 36 has a wider bandgap than the GaN layer 34 and thisdiscontinuity in energy band gaps results in a free charge transfer fromthe wider band gap to the lower band gap material. Furthermore, in theGroup III-Nitride system, piezoelectric and spontaneous polarizationresults in a significantly higher charge density. A charge accumulatesat the interface between the two layers and creates a two dimensionalelectron gas (2DEG) 35 that allows current to flow between the sourceand drain contacts 38, 40. The 2DEG 35 has very high electron mobilitywhich gives the HEMT a very high transconductance. An AlN spacer/barriermay also be included between the AlGaN layer and the GaN highresistivity layer, as disclosed in Patent Application Publication No. US2002/0167023 A1.

During operation, the drain contact 40 is biased at a specifiedpotential (positive drain potential for an n-channel device) and thesource is grounded. This causes current to flow through the channel and2DEG, from the drain to the source contacts 38, 40. The flow of currentis controlled by the bias and frequency potentials applied to the gate42, which modulate the channel current and provide gain. The voltageapplied to the gate 42 electrostatically controls the number ofelectrons in the 2DEG directly under the gate 42, and thus controls thetotal electron flow.

A bonding pad 43 is also included on the source contact 38 for flip-chipbonding to the circuit substrate as described below. When the HEMTs 32on the wafer 30 are separated into individual HEMTs, portions of the GaNlayer 34 and SiC wafer 30 between the HEMTs are removed leavingindividual devices as shown in FIG. 4. In alternative configurations,the wafer 30 upon which the device active layers and terminals 32 aredeposited may be formed from other semi-insulating materials such assapphire, Si or GaN. The wafer 30 may also be formed from an insulatingmaterial such as AlN or a ceramic material.

FIGS. 5-8 show different embodiments of circuit substrates according tothe present invention, although other circuit substrates can also beused. FIG. 5 shows a circuit substrate 50 according to the presentinvention, which includes a wafer 51 that can be made of many differentmaterials including GaAs. The wafer 51 has passive components andinterconnects 53 deposited on its top surface. The wafer 51 can havemany different thicknesses, with a suitable thickness being in the rangeof 50 to 500 micrometers. Wafers of other material can also be used,including Si, SiC, InP, AlN and ceramic, with the preferred wafers beingeasy to process, having low electrical conductivity and/or having highthermal conductivity.

Different passive components can be used including resistors 56 orcapacitors 58, and the interconnects 53 can be conductive traces 60. Thepassive components 52 and the interconnects 53 together serve as thedrive electronics and matching circuit for an active device that isflip-chip mounted on substrate 50 (as described below). The substrate 50can have drive electronics for more than one active device and thepassive components 52 and traces can be formed using the methods asdescribed above in the method of FIG. 1.

A hole 61 is formed through the GaAs wafer 51, and the inside surfaceand top opening of the hole is covered with a layer 62 of materialhaving high electrical and thermal conductivity. The layer 62 forms aconductive via 63 through the wafer 51. The bottom surface of the wafer51 can also be covered with a layer of material 64 having highelectrical and thermal conductivity, with the preferred material for thelayers 62 and 64 being Au. Electrical current and heat pass through thelayer 62 and are spread into layer 64. The layers 62 and 64 togetherserve as the electrical contact to ground for a device that is flip-chipmounted on the substrate 50 and they also help dissipate heat from theflip-chip mounted device. This is particularly useful for GaAs and Sisubstrates that have relatively low thermal conductivity. Generally, thelarger the vias 63 the more efficient the circuit substrate 50 atdissipating heat, if the circuit substrate does not have higher thermalconductivity than the material in the vias. Typical vias are 50-100microns wide, but wider or narrower vias can be used.

FIG. 6 shows a circuit substrate 70 according to the present invention,which includes a GaAs wafer 71 that is similar to the wafer 51 in FIG.5, and can be made of the same materials. The wafer 71 has passivecomponents 72 and interconnects 73 deposited on its top surface to formdrive electronics. However, the wafer 71 does not have a hole, aconductive via or a conductive layer on its bottom surface. Instead, toprovide a conductive path to ground a conductive trace 74 is included onthe surface of the wafer 71. Devices can be flip chip mounted on thewafer 71 with the device ground connection to the trace 74, such that aconductive path through the wafer 71 for ground or heat dissipation isnot needed.

FIG. 7 shows a circuit substrate 80 according to the present inventionthat is similar to the circuit substrate 50. It includes a wafer 81 anda conductive vias 82 formed in the hole 88. However, in substrate 80 aplug 84 of material with high electrical and thermal conductivity isincluded. The plug 84 is at the top of the hole 88, with the plugs topsurface at the top surface of the wafer 81. The vias layer 86 covers theinside of the hole 88 and the bottom surface of the plug 84. The plug 84is preferably made of gold (Au) and allows the substrate 80 to moreefficiently conduct heat away from the flip-chip device mounted on it.The substrate 80 also includes passive components 85, interconnects 83,and bottom conductive layer 89.

FIG. 8 shows another embodiment of a circuit substrate 90 according tothe present invention that includes a wafer 91, and passive components92 that can include but are not limited to a capacitor 94, resistor 96and interconnects 97. In addition, the substrate 90 has pre-stageamplifiers 98 a and 98 b that can be made of many material systemsincluding InGaAs and InP. The amplifiers 98 a, 98 b serve as pre-stagesof amplification are typically connected in series to amplify lowersignals. After the pre-stage amplifiers 98 a, 98 b amplify the signal,it is applied to the amplifier flip-chip mounted on the substrate 90,for high power amplification. The pre-stage amplifiers 98 a, 98 b arepreferably HEMTs and 2 to 3 pre-stage amplifiers are typically used,although more or less could also be used. As outlined above, thepre-stage amplifiers 98 a, 98 b are formed by a transistor and passivecomponents 92, each of which can be fabricated on the circuit substrate91 using a commercial foundry.

A circuit substrate according to the present invention can have anycombination of the substrate features shown in FIGS. 5-8. For instance,the substrate can have pre-stage amplifiers without vias, or if thesubstrate has vias, the vias can be used without a plug. Accordingly,there are many additional embodiments of the circuit substrate accordingto the present invention beyond those described above.

FIG. 9 shows a flip-chip integrated circuit (IC) assembly 100 accordingto the present invention, having the HEMT 32 of FIG. 4 flip-chip mountedon the circuit substrate of FIG. 7, although the HEMT 32 can also beflip-chip mounted on the circuit substrates 50, 70 and 90 in FIGS. 5, 6and 8. The same reference numerals from FIGS. 4 and 7 are used for thesame features.

The HEMT's bonding pad 43 on the source contact 38 is bonded to thesurface of the circuit substrate 80 with the bonding pad 43 over theplug 84. The bonding pad 43 is bonded in electrical and thermal contactwith the plug 84. The layer 89 serves as the integrated circuit's groundand the source contact 38 is connected to layer 89 through the plug 84and via layer 86. Heat also flows to the layer 89 from the HEMT 32through the plug 84 and via layer 86. The drain 40 and gate 42 areconnected to the passive components 82 through conductive connections102, 104 and interconnects 83 on the circuit substrate 80. By flip-chipmounting as shown in FIG. 9, integrated circuits 100 can be manufacturedat lower cost with higher yield.

FIG. 10 shows an IC assembly 110 that is similar to the integratedcircuit 100 in FIG. 9, but has improved heat dissipationcharacteristics. It has the same HEMT device 32 that is flip-chipmounted to a circuit substrate 80 having passive components 82 andinterconnects 83. A first heat sink base plate 114 is disposed adjacentto the conductive layers 86 and 89, with heat from the layers flowinginto the first base plate 114. Heat from the first base plate 114 thenflows into an external heat sink (not, shown) where it is dissipated.The base plate 114 and heat sink should be made of a thermallyconductive material that conducts heat away from the substrate 80 andthe HEMT 32, with suitable materials being Cu, Cu—W, Cu—Mo—Cucomposites, AlN, diamond or other conventional heat sink materials. Thebase plate 114 and heat sink help keep the HEMT 32 from overheating athigher power levels. The base plate 114 can also be used with a circuitsubstrate that does not have a conductive plug.

FIG. 11 shows another IC assembly 120 according to the present inventionthat is similar to the IC assembly 110 in FIG. 10, but includesadditional heat dissipation features. The IC assembly 120 has a HEMT 32flip-chip mounted on a circuit substrate 80 that has the same first baseplate 114 as the circuit 110 in FIG. 10. To improve heat dissipationthrough the HEMT's substrate 30, the IC assembly 120 also has a secondheat sink base plate 122 arranged adjacent to the SiC substrate 30. Thesecond base plate 122 is coupled to a second heat sink (not shown),which provides another path for heat from the HEMT 32 to dissipate. Thesecond base plate 122 and second heat sink can be made of the same ordifferent materials as the first base plate 114 and heat sink, butshould be made of a heat conductive material. The second base plate 122can also be used with a HEMT that is flip-chip mounted on circuitsubstrates that does not have a plug 84. It can also be used in an ICassembly that does not have a first heat sink, although the mostefficient heat dissipation is provided by using first and second baseplates 114, 122 with their respective heat sinks. In other embodimentsof the invention, the second base plate 122 can be the primary path forheat dissipation and/or the first base plate 114 can be simplified oromitted. To compensate for thermal expansion of the IC assembly 120, thefirst or second base plates 114, 122 can be connected to a thermallyconductive encapsulation instead of a heat sink, with the encapsulationbeing somewhat resilient or flexible.

FIG. 12 shows an IC assembly 130 according to the present inventionhaving a flip-chip mounted GaN HEMT 131 mounted on a circuit substrate132. Passive components, pre-stage amplifiers and interconnects (notshown) can be included on the substrate 132. A top heat sink base plate133 is included on the HEMT 131, with the base plate 133 also mounted tothe substrate 132 by spacers 134 that can be dummy chips or solderbumps. The spacers 134 arrangement holds the heat sink base plate 133adjacent to the HEMT 131 while providing a stable attachment of the baseplate 133 to the substrate 132.

FIG. 12 a shows a semiconductor assembly 200 having dual heatdissipation capabilities. The assembly includes an active semiconductordevice 202, such as a GaN or SiC transistor similar to those describedabove. The semiconductor device 202 includes a gate terminal G, sourceterminal S and a drain terminal D on a first surface and a layer ofthermally conductive material 204 on a second surface opposite the firstsurface. The transistor 202 is flip chip mounted on what is effectivelya thermally conductive substrate 206. This thermally conductivesubstrate 206 may include, for example, a GaAs material with vias asshown in FIG. 5, a GaAs material with vias and plugs as shown in FIG. 7or a material having high thermal conductivity, such as SiC, InP, AlN orceramic. The use of a high thermal conductivity material may reduce oreliminate the use of vias. A first heat sink 208 is arranged adjacentthe thermally conductive substrate 206 while a second heat sink 210 isadjacent the layer of thermally conductive material 204 portion of thetransistor.

The heat sinks used in any of the assemblies described in FIGS. 10-12 a,may be made from any high thermal conductivity material. Examples ofsuch materials include metals, such as aluminum and copper; inorganicmaterials, such as carbon and ceramics; and composite materials. Theheat sinks may also be formed using porous configurations of any ofthese materials. One such material is Durocel™ porous aluminum foamwhich is available from ERG Material and Aerospace Corporation.

The flip-chip integrated circuits described may be used to implementvarious types of circuitry including RF, microwave and mmwave poweramplifiers. FIG. 13 is a general schematic of an amplifier circuitincluding various amplifier electronics that form a multistagepreamplifier section 140 having an input 141 and an output 142. Whilethe preamplifier section 140 includes two amplifiers connected inseries, additional preamplifiers and/or other circuit components may beincluded. Each preamplifier includes one or more passive circuitelements 143 interconnected with a transistor 144. The amplifier circuitalso includes an output stage amplifier 145 which also includes atransistor 146 and passive circuit elements 147. The output 142 of themultistage preamplifier 140 is input to the output stage amplifier 145.In accordance with the present invention, the components of themultistage preamplifier section, i.e., the active transistors 144 andpassive circuit elements 143, and the passive components 147 of theoutput stage amplifier 145 may be included in a circuit substrate whilethe active transistor 146 of the output stage amplifier may beseparately formed and flip-chip mounted on the circuit substrate.

FIG. 14 shows an example of an integrated circuit assembly that realizesaspects of the schematic of FIG. 13. To preserve clarity of illustrationonly some of the components of FIG. 13 are shown in FIG. 14. Whereapplicable, correlation between the two figures is provided in thefollowing description of FIG. 14 by parenthetical reference to elementsof FIG. 13.

The assembly includes a circuit substrate 150 with passive circuitelements including a resistor 151 (FIG. 13, 143) and a capacitor 152(143), interconnections 153 and an active transistor 154 (FIG. 13, 144),formed thereon. The circuit substrate 150 may be designed and fabricatedin a foundry process including front side processes, e.g., activetransistors, passive circuit elements like transmission lines,resistors, capacitors etc., and backside processes, e.g., via holes andground plane. In a preferred embodiment, the circuit substrate 150 is aGaAs based substrate and the transistor 154 is a GaAs transistor. Inother configurations, the circuit substrate 150 may be a Si based, SiCbased, InP based, AlN based or ceramic based substrate. The circuitsubstrate 150 may include the pre-amplifier 140 or driver stage of theamplifier circuit of FIG. 13.

With continued reference to FIG. 14, the substrate 150 includesconductive vias 155 that connect a first surface 156 of the circuitsubstrate with a second surface 157 of the substrate. A conductivestructure 158, such as a ground plane, is adjacent the second surface157 and in electrical connection with the vias 155. The ground plane 158provides the ground connections required by the passive circuit elements151, 152 and the transistors 154 forming the multistage preamplifiersection. In an alternate configuration, the ground connection may beprovided by an interconnection, or trace, on the first surface 156 ofthe circuit substrate. Such a configuration, sometimes referred to as a“coplanar waveguide” configuration, essentially eliminates the need forconductive vias 155.

With continued reference to FIG. 14, the assembly also includes aflip-chip mounted transistor 159 (FIG. 13, 146) that is fabricatedseparately on a suitable substrate 160 such as SiC. In a preferredembodiment, the transistor 159 is Group III nitride transistor such as aGaN HEMT configured as shown and described previously with reference toFIG. 4. The transistor 159 is mounted at the outputs stage of thecircuit fabricated on the circuit substrate. In particular, thetransistor 159 is mounted such that the source contact 161 iselectrically connected to the ground plane 158 through a conductive via155, the gate contact 162 is connected to the output of the prestageamplifier section (not shown) (FIG. 13, 142) and the drain contact 163is connected to passive circuit elements (not shown) (FIG. 13, 147) onthe circuit substrate. The transistor 159, in combination with thepassive circuit elements, forms a “flip-chip” amplifier.

The use of a GaN HEMT based amplifier at the output stage can providegreater than 10 times the output power compared to that available froman equivalent GaAs output stage. The flip-chip process takes advantageof the fact that the rest of the circuit, including GaAs amplifierbefore the output stage, interstage matching, and circuit environment,is all completed in a commercial process on large area substrates. Anintegrated circuit configured as such allows for the heterogeneousintegration of multiple semiconductor technologies. For example, such acircuit may utilize a commercial lower cost GaAs MMIC multistagepreamplifier and a high output stage GaN HEMT amplifier to provide acircuit having an increased total power output.

In addition to RF and microwave circuits, the flip-chip integratedcircuits described may be used in the area of power electronic circuits.Typical power electronic circuits, such as power converters, invertersand switching circuits, are implemented with Si based electronics. Alsopower electronic circuits typically utilize extensive control circuitsthat are most easily and economically implemented with Si CMOS. Thesecontrol circuits provide appropriate signals and synchronization tovarious nigh power elements such as switching transistors. In accordancewith the present invention, a GaN HEMT power switching transistor may beused in conjunction with the Si based electronics to provide a circuithaving an extremely high breakdown voltage (provided by the Group IIInitride transistor), high mobility and very low ON resistance.

FIG. 15 is a general schematic of a power electronics circuit includinga switching device 170, e.g., transistor, a gate control circuit 171 anda drain control circuit 172. In accordance with the present invention,the components of the control circuits 171, 172, e.g., logic circuits,may be included in a circuit substrate while the switching device, i.e.,active transistor 170 may be separately formed and flip-chip mounted onthe circuit substrate.

FIG. 16 shows an example of an integrated circuit assembly that realizesaspects of the schematic of FIG. 15. To preserve clarity of illustrationonly some of the components of FIG. 15 are shown in FIG. 16. Whereapplicable, correlation between the two figures is provided in thefollowing description of FIG. 16 by parenthetical reference to elementsof FIG. 15.

The assembly includes a circuit substrate 180 having passive circuitelements including a resistor 181 and a capacitor 182, interconnections183 and logic circuitry 184 formed thereon. In a preferred embodiment,the circuit substrate 180 is a Si based substrate and the logiccircuitry 184 is Si logic. In other configurations, the circuitsubstrate 180 and logic circuitry 184 may be formed using a differentsemiconductor material, such as SiC. The substrate 180 includesconductive vias 185 that connect a first surface 186 of the circuitsubstrate with a second surface 187 of the substrate. A conductivestructure 188, such as a ground plane, is adjacent the second surface187 and in electrical connection with the vias 185. The ground plane 188provides the ground connections required by the passive circuit elements181, 182 and logic circuitry 184. Alternatively, the circuit substrate180 may have a coplanar waveguide configuration, as previous described.

With continued reference to FIG. 16, the assembly also includes aflip-chip mounted transistor 189 (FIG. 15, 170) that is fabricatedseparately on a suitable substrate 190 such as SiC. In a preferredembodiment, the transistor 189 is a GaN HEMT configured as shown anddescribed previously with reference to FIG. 4. In other configurations,the transistor 189 may be formed from other Group III nitrides or SiC.The transistor 189 is mounted such that the source contact 191 iselectrically connected to the ground plane 188 through a conductive via185, the gate contact 192 is connected to the output of the gate controlcircuit (not shown) (FIG. 15, 171) on the circuit substrate and thedrain contact 193 is connected to output of the drain control circuit(not shown) (FIG. 15, 172), also on the circuit substrate. In analternate configuration, the transistor 189 may be hardwired to thecircuit substrate, as opposed to being flip-chip mounted.

Although the present invention has been described in considerable detailwith reference to certain preferred configurations thereof, otherversions are possible. The sequence of the steps in the methodsdescribed above can be different. Other methods according to theinvention can use more or less steps and can use different steps. Allembodiments described above can be used with circuit substrates with orwithout pre-stage amplifiers and with or without via plugs. Manydifferent types of integrated circuits made of many different materialscan be flip-chip mounted according to the invention. Therefore, thespirit and scope of the claims should not be limited to the versions onthe invention described in the specification.

1. A flip-chip integrated circuit comprising: a circuit substrate havingdrive electronics on a first surface and conductive material on a secondsurface opposite the first surface; an active semiconductor devicecomprising a plurality of terminals on a first surface and a layer ofmaterial on a second surface opposite the first surface, the activesemiconductor device flip-chip mounted on the circuit substrate, atleast one of the terminals in electrical contact with the driveelectronics on the circuit substrate; a first heat sink adjacent theconductive material on the circuit substrate; and a second heat sinkadjacent the layer of material on the active semiconductor device. 2.The circuit of claim 1, wherein the layer of material is thermallyconductive.
 3. The circuit of claim 1, wherein the layer of material isa semiconductor.
 4. A flip-chip assembly comprising: a thermallyconductive substrate; an active semiconductor device comprising aplurality of terminals on a first surface and a layer of thermallyconductive material on a second surface opposite the first surface, theactive semiconductor device flip-chip mounted on the thermallyconductive substrate; a first heat sink adjacent the thermallyconductive substrate; and a second heat sink adjacent the layer ofthermally conductive material on the active semiconductor device.
 5. Theassembly of claim 4, wherein the heat sink is formed from a high thermalconductivity porous material.
 6. The assembly of claim 5, wherein theporous material is a metal.
 7. The assembly of claim 6, wherein theporous material is an inorganic material.
 8. A method of fabricating anintegrated circuit, comprising forming an active component and aplurality of passive circuit elements on a first surface of a circuitsubstrate; forming separately from the circuit substrate, an activesemiconductor device comprising layers of material and a plurality ofterminals, each of the terminals in electrical contact with one of thelayers of material; and flip-chip mounting the active semiconductordevice on the circuit substrate, at least one of the terminalselectrically connected to the active component on the circuit substrate.9. A method of fabricating an integrated circuit, comprising: formingamplifier electronics and passive circuit elements on a first surface ofa circuit substrate; forming a semiconductor transistor separately fromthe circuit substrate; and flip-chip mounting the transistor on thecircuit substrate, the transistor in electrical connection with passivecircuit elements on the first surface, and forming, in combination withpassive circuit elements, a flip-chip amplifier having an inputelectrically connected to the amplifier electronics.
 10. A method offabricating an integrated circuit, comprising: forming control circuitson a first surface of a circuit substrate; forming separately from thecircuit substrate, a semiconductor transistor having a plurality ofterminals; and flip-chip mounting the transistor on the circuitsubstrate, at least one of the transistor terminals electricallyconnected to the control circuits on the first surface.
 11. A method offabricating an integrated circuit, comprising: forming control circuitelements on a first surface of an Si based circuit substrate; formingseparately from the circuit substrate, a Group III nitride basedsemiconductor transistor having a gate and a drain; and mounting thetransistor on the circuit substrate, at least one of the gate and drainelectrically connected to one of the control circuit elements on thefirst surface.
 12. A method of fabricating an integrated circuit,comprising: forming passive circuit elements on a first surface of acircuit substrate comprising an insulating material; forming separatelyfrom the circuit substrate, a Group III nitride base activesemiconductor device; and flip-chip mounting the semiconductor device onthe circuit substrate, the device electrically connected to the passivecircuit elements.